Quantum cascade lasers provide a tunable mid-infrared (MIR) light source that can be used for spectroscopic measurements and images. Many chemical components of interest have molecular vibrations that are excited in the MIR region of the optical spectrum, which spans wavelengths between 2.5 to 25 microns. Hence, measuring the absorption of MIR light at various locations on a sample can provide useful information about the chemistry of the sample as a function of position on the sample.
One class of imaging spectrometers measures the light directly reflected from the sample as a function of position on the sample and wavelength of the illuminating MIR light. The amount of light that is reflected depends on both the chemical and physical attributes of the sample, since light can be lost both by absorption in the sample, which reflects the chemical composition of the specimen and by scattering, which depends on the physical state of the surface of the specimen. Hence, comparing spectra generated with direct reflection to absorption with known chemical absorption spectra that are available in libraries presents significant challenges.
Systems that utilize attenuated total reflection (ATR) to illuminate the specimen avoid the problems caused by scattering of the incident light by the specimen. For example, U.S. Pat. No. 9,863,877, issued Jan. 9, 2018 describes a scheme for scanning a portion of a specimen using ATR. These schemes reflect an incident light beam from a crystal surface at an angle that is less than the critical angle. The reflected light intensity is measured and compared to the incident light intensity to determine the absorption provided by a specimen that is in contact with the reflecting surface, but outside the crystal. While the light is totally reflected, the electric field generated by the light extends a few microns outside of the reflecting surface and can interact with the specimen. If the specimen absorbs light of the wavelength in the incident beam, the reflected light will be attenuated.
These schemes require that the specimen be brought within a few microns of the reflecting surface, and preferably, in contact with the reflecting surface. Problems arise when the specimen is either fragile or hard. The user must move the specimen such that the specimen touches the reflecting surface without forcing the specimen against the reflecting surface with sufficient force to damage either the specimen or the crystal. In practice, moving the specimen into position takes several minutes, and hence, limits the throughput of the spectrometer and requires significant operator skill to achieve the desired result.